1. Field of the Invention
The present invention generally relates to a liquid crystal medium useful as a material for an optical device, in particular, to a liquid crystal medium having a wide temperature range of a liquid crystal phase, a large dielectric anisotropy, and an optical anisotropy. Additionally, the present invention further relates to an optical device using the liquid crystal medium, and in particular, to an optical device that can be used in a wide temperature range and driven at a low voltage, and is capable of achieving a high-speed electro-optical response.
2. Description of Related Art
Liquid crystal display (LCD) devices utilizing liquid crystal compositions are widely used in displays of clocks, calculators, word processors, and the like. These LCD devices utilize optical anisotropy, dielectric anisotropy and the like of liquid crystal compounds. The operation modes of the LCD devices are known to mainly include phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), and vertical alignment (VA) and the like, which utilize one or more polarizers for display. Moreover, in recent years, more attentions have been paid to the mode where an electric field is applied to an optically isotropic liquid crystal phase to enable the liquid crystal phase to exhibit electrically controlled birefringence (Patent References 1-9, and Non-patent References 1-3).
Moreover, wavelength variable filters, wavefront control elements, liquid-crystal lenses, aberration correction elements, aperture control elements, and optical head apparatuses and the like utilizing the electrically controlled birefringence in a blue phase which is one of the optically isotropic liquid crystal phases have been proposed (Patent References 10-12).
According to the driving mode, the devices are classified into passive matrix (PM) and active matrix (AM) types. The PM type is further classified into static type and multiplex type, and the AM type is further classified into thin film transistor (TFT) type and metal-insulator-metal (MIM) type.
Such LCD devices contain a liquid crystal composition having suitable physical properties. In order to improve the characteristics of an LCD device, it is preferred that the liquid crystal composition has suitable physical properties. A liquid crystal compound as a component of the liquid crystal composition needs to have the following general properties:
(1) stable chemical properties and stable physical properties;
(2) a high clearing point (i.e., liquid crystal phase-isotropic phase transition temperature);
(3) a low lower-limit temperature of a liquid crystal phase (i.e., nematic phase, cholesteric phase, smectic phase, and optically isotropic liquid crystal phase such as blue phase);
(4) excellent compatibility with other liquid crystal compounds;
(5) a suitable dielectric anisotropy; and
(6) a suitable optical anisotropy.
Particularly, for an optically isotropic liquid crystal phase, a liquid crystal compound having a large dielectric anisotropy and a large optical anisotropy is preferred, in view of lowering the driving voltage.
When a liquid crystal composition containing a liquid crystal compound having stable chemical and physical properties as described in (1) is used in an LCD device, the voltage holding ratio is improved.
Further, a liquid crystal composition containing a liquid crystal compound having a high clearing point or a low lower-limit temperature of a liquid crystal phase as described in (2) and (3), may have an expanded temperature range of a nematic phase or an optically isotropic liquid crystal phase, thus being used in display devices in a wider temperature range. A single liquid crystal compound exhibits properties which are difficult to function, thus it is generally used in mixture with a number of other liquid crystal compounds to prepare a liquid crystal composition. Therefore, a liquid crystal compound having good compatibility with other liquid crystal compounds as described in (4) is preferably used in LCD devices. In recent years, LCD devices with superior display performance, such as, contrast, display capacity, and response time, are especially required in the industry. In addition, as for the liquid crystal material used, a liquid crystal composition having a low driving voltage is required. Furthermore, in order to drive optical devices driven in an optically isotropic liquid crystal phase with a low voltage, a liquid crystal compound with a large dielectric anisotropy and a large optical anisotropy is preferred.
As for the optically isotropic polymer/liquid crystal composite materials disclosed in Patent References 1-3 and Non-patent References 1-3, the voltages needed for operation of the devices are high. Although optically isotropic liquid crystal compositions and polymer/liquid crystal composite materials expected to have an operation voltage lower than the voltages described above are disclosed in Patent References 4-9, an optically isotropic liquid crystal composition and a polymer/liquid crystal composite material containing a compound having a pyrimidine ring and a linking group —CF2O— of the present application have not been reported.